The virus which causes smallpox can only live in man. Once people are infected, either they die or the virus dies with them, or the immune system of the body kills the virus and the victim survives, or one infected person infects another.

The virus which causes smallpox can only live in man. Once people are infected, either they die or the virus dies with them, or the immune system of the body kills the virus and the victim survives, or one infected person infects another.

Smallpox vaccination with vaccinia virus (which is not the same as variola virus which causes smallpox) protects against infection. Thus, if an infected person surrounded by vaccinated people, the virus has nowhere to go and dies out. In larger populations the virus dies out when it is isolated by being surrounded by vaccinated persons or by survivors who have had the disease and have become immune.

There are no subclinical cases of smallpox-you either have it or you don’t. It must be passed on from one person to another at least every three weeks or dies it out. That would mean an absolute minimum of almost 20 cases per year, and smallpox just doesn’t go unnoticed.

So when competent worldwide surveillance system cannot detect anyone with smallpox, we can be sure it doesn’t exist. And if it doesn’t exist, it cannot come back.